Heterobilharzia americana

Heterobilharzia americana is a species of blood fluke which is native to North America. It is mainly a parasite of raccoons, but is also capable of infecting a wide range of other mammals. Its broad taste for different hosts brings them into contact with various domesticated animals, in particular, dogs. Indeed, it is more commonly known as the canine schistosome.

Left: Cercaria of H. americana, Right: Adults (left = male+female pair, right = single female fluke) 
Photos from Fig. 2 and Fig. 4 of the paper

When the aquatic larvae of these flukes come into contact with humans, much like those of bird schistosomes,  they get intercepted and killed by the immune system as they burrow into the skin, and the death throes of these larval parasites manifest themselves as an itchy rash. But in dogs, not only is the parasite able to establish itself and grow to sexual maturity, it also causes far more severe symptoms than merely an itchy rash.

Dogs infected with H. americana exhibit a host of serious pathologies including vomiting, bloody diarrhoea, weight loss, and lethargy. Additionally, the vast number of microscopic eggs produced by the adult flukes are transported via the circulatory system to various internal organs where they can cause inflammation, and form pockets of mineralization. While the parasite can be treated with prazinquantel and fenbendazole, their presence often go under-diagnosed as the disease they manifest are rather nonspecific, and the existence of this parasite is not as commonly known. 

Some of those eggs that are carried in the circulatory system eventually make their way to the outside world via the host's faeces. If an egg reaches a water body such as a pond, it hatches into a ciliated larva which seeks out a suitable snail host. Snails play an important role in the life cycle of digenean flukes like H. americana, for this is where asexual reproduction takes place. Through commandeering much of the snail's internal organs, the parasite raises an entire clone army of free-swimming larval stages called cercariae. A single infected snail can produce and release hundreds of infective cercariae into the surrounding waters on a regular basis.

The usual snail host for H. americana is Galba cubensis, a pond snail mostly found in the warmer parts of the Americas including Mexico, South America, and south-eastern parts of the United States. But a new study indicates that it has recruited a new snail host for the asexual stage of its life cycle, one which would allow it to spread further across North America. 

This study was based on a two-year long investigation into a small outbreak of H. americana in east-central Moab, Utah, where two severely ill dogs were euthanised after exhibiting symptoms associated with canine schistosomiasis. A necropsy revealed many of their internal organs were riddled with inflammation and mineralization caused by the presence of blood fluke eggs. Examination of faecal samples from other dogs in the neighbourhood found that some of them also contained the parasite's eggs.

Given the life cycle of H. americana, the researchers determined that the most likely source of the infection was a nearby irrigation pond which was regularly visited by dogs living in the neighbourhood, including the two deceased dogs. The pond was filled with many different species of aquatic snails, but there was just one species that was shedding schistosome cercariae - an amphibious snail called Galba humilis, which lived along the banks and waterline of the pond.

Galba humilis is widely known to serve as a host for liver flukes (Fasciola hepatica),  but this is the first time it has been recorded to host the canine schistosome as well. While these tiny snails are barely a centimetre in length, each can produce thousands of infective cercariae over its lifetime. The researchers found that on average, an infected snail can release about 800 cercariae during each shedding period. Furthermore, to increase their chances of encountering a host, they mostly come out of the snail at night between 6:00 pm and 7:30 pm, which overlaps with the active period of their main mammalian hosts - raccoons. 

Based on the faecal samples the researchers found in the area, the animals which introduced H. americana to the snails in that pond in Moab were most likely to be the aforementioned raccoons. And they were able to confirm this via experimental infection of snails from a captive-raised colony.

The most worrying implications from this study is that by acquiring a snail like G. humilis as a host, H. americana would be able to spread to more temperate regions. Galba humilis is widely found across the United States and is a common snail in human-built habitats like irrigation dams and ponds. These habitats also attract various animals like raccoons and other mammals which are viable hosts for this parasite. Thus these water bodies can bring together everything H. americana needs to complete its life cycle and reproduce.

With landscape changes due to agriculture, urbanisation and climate change, raccoons have become more abundant and are expanding their range across North America. In addition to raccoons, dogs are also common in urban areas and can serve as key reservoirs and means of dispersal for the parasite. Whenever they visit a pond with snails, infected dogs can introduce and establish a new H. americana hotspot for the local mammal population.

All these factors contribute to further the spread of this parasite across North America, and possibly elsewhere too. Raccoons have become a prolific invasive species in many parts of the world, and since H. americana has already switched its snail host once, it may do it again to whatever amphibious or aquatic lymnaeid snail it encounters. For the canine schistosome and other parasites, what played out at that pond in Moab is a sign of things to come in our changing world.

Reference:

Loker, E. S., et al. (2021). An outbreak of canine schistosomiasis in Utah: Acquisition of a new snail host (Galba humilis) by Heterobilharzia americana, a pathogenic parasite on the move. One Health, 100280.

October 13 - Nanophyetus salminicola

West of the Cascade Range in the Pacific Northwest, it is an extremely poor idea to feed fish scraps to your dog. This tiny intestinal fluke is the reason. The parasite itself is relatively harmless but it can act as a vector for a rickettsial illness caused by Neorickettsia helminthoeca (“helminth,” as we all should know by now, is Greek for “worm”). When a canid is infected by eating uncooked salmon, the rickettsia can cause symptoms such as vomiting, diarrhea, and dehydration, which can be fatal in as many as 90% of cases if untreated. For unknown reasons, cats, raccoons and bears are not susceptible to the illness.

Contributed by Mike Kinsella.

October 3 - Toxocara canis

Toxocara canis is a roundworm (nematode) that occurs worldwide in dogs. In adults, they are generally asymptomatic, but they can be fatal to puppies, particularly if the infection is heavy. Transmission usually occurs via ingestion of the eggs, which are passed in feces. Once swallowed, the larvae hatch and embark on a scenic route - from the intestine to the liver and lungs via the blood stream, where they are coughed up, swallowed again, and eventually mature. Other hosts, including human pet-lovers, can become infected and that characteristic larval wandering can make those humans very sick themselves.

July 14 - Angiostrongylus vasorum

Angiostrongylus vasorum is commonly known as the "French heartworm", though unlike Dirofilaria immitis and D. ursis heartworms, this parasite is not a mosquito-borne filarial worm. Instead, this is a strongylid nematode, so like Strongyloides westeri. Both domestic and wild canids, especially foxes, are the vertebrate hosts for this parasite, but unlike the name suggests, the parasites are not purely confined to France, but are rather broad geographically. As you might guess, though, the adult worms reside in the host's heart. The eggs lodge themselves into lung capillaries and eventually the hatching larvae break through into the lung tissue where they get coughed up and swallowed, and passed out through the feces.

Image is from this site.

July 13 - Dioctophyma renale

The aptly named Dioctophyma renale reaches maturity in the host’s kidneys. Also known as giant kidney worms, they are one of the largest nematodes capable of infecting humans-adult females grow to be over 100 cm long (they are also bright red and about 10 mm wide). Although they don’t feed on kidney tissue, they cause pressure necrosis (imagine a meter-long, centimeter wide worm living in your roughly computer mouse-sized kidney), turning the kidney into a non-functioning hollow shell. It has a cosmopolitan distribution and affects humans, several species of canids, horses and pigs, and uses annelids as intermediate hosts (fish can be paratenic hosts). Vertebrate hosts become infected by eating an infected intermediate or paratenic host or by drinking water in which they are swimming. Luckily, human infections are uncommon. On rare occasions, they become encysted in other organs (e.g. ovaries, uterus, urethra, mammary gland or subcutaneous tissue) or migrate through the digestive tract or abdominal cavity, making for an unpleasant finding during veterinary surgical procedures.

Contributed by Andrés Gómez.
Photo from this site.

March 7- Dipylidium caninum

Yesterday, you met the cat flea. Believe it or not, these tiny blood-feeding insects are the intermediate hosts to a tapeworm that as adults can grow up to 18 inches (or 40-50 cm) long once they are ensconced in the intestine of their mammalian host! The eggs of the tapeworm are expelled in the host’s feces and are ingested by fleas. Inside the flea, the eggs hatch and the larval tapeworms travel into the body cavity and wait there for a mammal to eat the flea – usually when they are grooming an itchy spot. Children can become infected with D. caninum if they accidentally (or purposefully?) ingest infected fleas.

February 1 - Echinococcus granulosis

When people think of tapeworms, they often think of very long ones, like Taenia saginata. Echinococcus granulosis is a tiny tapeworm – tiny, but nasty. The main vertebrate hosts of E. granulosis are canids, where the 5 millimeter-long adult tapeworms live in the small intestine. Eggs are expelled in the dog’s feces, where they are eaten by herbivores such as sheep or deer or rodents. Inside the herbivore, the larvae travel through the intermediate host’s blood and take residence in various organs where they form hydatid cysts, which can grow very large in some cases – as big as a grapefruit or even larger. It is thought that these cysts make the herbivore more vulnerable to predation by – canids, of course. Humans can serve as intermediate hosts if they are exposed to contaminated dog (or coyote or wolf or other canid) feces and will suffer from hydatid disease when the larval tapeforms form cysts in our organs. This can be a serious condition, not only because the large cysts can put pressure on organs, but also because should the cysts rupture within the body, a person can suffer from severe shock. The disease can be common in areas with many sheep.

January 30 - Dermacentor variabilis

Ticks are small, blood-feeding arachnids, relatives of spiders and mites. The American dog tick, Dermacentor variabilis has three life stages: larvae (which are 6-legged not 8!), nymphs, and adults. As the ticks pass from each life stage to the next, they require a bloodmeal from a vertebrate. Larvae and nymphs will usually obtain this bloodmeal from rodents, but adults, as their name suggests will seek out larger mammals, particularly dogs, and sometimes humans. These ticks find their hosts using a behavior known as “questing”. They crawl to the top of a blade of grass or other variation and extend their front two legs until a host brushes past, at which point they grab on. The life cycle can take two years to complete, with the ticks overwintering in the soil. D. variabilis is another example of an ectoparasite that can vector other diseases, in this case, Rocky Mountain Spotted fever, tularemia, and ehrlichiosis, which are all bacterial zoonotic diseases.

Image from Insect Images (but remember that ticks are not insects!)

January 21 - Dirofilaria immitis

Heartworm, Dirofilaria immitis, is a filarial nematode parasite that usually infects dogs or other canines, but can also infect cats, foxes, ferrets, and sea lions. The adult worms live in the right ventricle of the heart, hence their common name. After the adults mate, they produce microscopic larvae known as microfilariae, which are ingested by mosquitos when they bite the mammalian host. The larvae complete their development in the mosquito and then are transmitted to a new host when she feeds again. Although most dogs do not show signs of infection, in some cases worms can cause issues and need to be treated. However the treatments are sometimes risky because the dead worms can get carried to the lungs and cause respiratory distress or failure. Preventative therapies, such as giving dogs the drug ivermectin, are effective and safe. Heartworm are also hosts to the endosymbiotic bacteria, Wolbachia (see January 12th).

Image from this website.